Commonly assigned U.S. Pat. No. 4,987,648 of R. L. Thornton, which issued Jan. 22, 1991 on "Lateral Heterojunction Bipolar Transistor (LHBT) and Suitability Thereof as Hetero Transverse Junction (HTJ) Laser," describes the use of impurity induced disordering (IID) for forming the p-n junctions of planar buried heterostructure compound semiconductor devices, including "dual function HTJ laser/L-HBT transistor devices" (i.e., devices which can be biased for operation either as HTJ lasers or as L-HBT transistors). Buried heterostructure devices of that general type are preferred because their internal structures are planar and are fabricated entirely by diffusion, without any intermediate etching steps or any other interruptions requiring epitaxial regrowth. The above-identified Thornton'648 patent hereby is incorporated by reference, together with the following related articles: R. L. Thornton et al., "Unified Planar Process for Fabricating Heterjunction Bipolar Transistors and Buried Heterostructure Lasers Utilizing Impurity Induced Disordering" Applied Physics Letters, Vol. 53, No. 26, Dec. 26, 1988, pp. 2669-2671; and R. L. Thornton et al., "Demonstration and Properties of a Planar Heterojunction Bipolar Transistor with Lateral Current Flow," IEEE Transcations on Electron Devics, Vol. 36, No. 10, October 1989, pp. 2156-2164.
Heretofore the accepted practice has been to bury the active region of a HTJ laser sufficiently far below the outer surface of the semiconductor to ensure that the evanescent optical field of the laser decays to a negligibly low intensity level before it reaches the outer surface of the semiconductor. For example, the active regions of AlGaAs HTJ lasers characteristically have been buried at depths in excess of about 1 .mu.m for effectively isolating their evanescent optical fields from their usual GaAs capping layers and from the metallizations that are normally provided for making external electrical connections to them. This practice has been followed, even though it was recognized that the performance of the structurally corresponding L-HBT transistor was adversely affected by burying its active base channel so deeply. More particularly, it was known that the width of the base channel of such a transistor could be significantly reduced and more precisely controlled, while maintaining a given photolithographic feature size, by forming the active base channel closer to the outer surface of the multilayer heterostructure. Furthermore, conventional HBT technology provided convincing evidence that L-HBT transistors having narrower base channels would have higher current gain and improved high frequency performance. Nevertheless, until now, the depth to which the active base regions of dual function HTJ laser/L-HBT devices have been buried has been governed by the assumption that the evanescent optical field intensity of the waveguided modes of the laser configuration of such a device should be neglibible at the surface of the device.